Conventionally, differential pressure sensors that incorporate pressure sensor chips that use sensor diaphragms for outputting signals in accordance with differences between pressures borne on one face and borne on the other face have been used as differential pressure sensors for industrial use. These differential pressure sensors are structured so as to guide the respective measurement pressures, which will act on high-pressure-side and low-pressure-side pressure bearing diaphragms, to one side face and the other side face of a sensor diaphragm, through a filling liquid as a pressure transmitting medium, so as to detect the deformation of the sensor diaphragm as, for example, a change in a resistance value of a strain resistance gauge, to convert this change in the resistance value into an electric signal, so as to be outputted to the outside.
This type of differential pressure sensor is used when measuring, for example, a liquid surface height through detecting a pressure difference between two locations, upper and lower, within a sealed tank for storing a fluid that is to be measured, such as a high-temperature reaction tower in an oil refining plant.
FIG. 10 is illustrates a schematic structure for a conventional differential pressure sensor. This differential pressure sensor 100 is structured through incorporating, in a meter body 2, a pressure sensor chip 1 having a sensor diaphragm (not shown). The sensor diaphragm in the pressure sensor chip 1 is made from silicon, glass, or the like, and a strain resistance gauge is formed on a surface of the diaphragm, which is formed in a thin plate shape. The meter body 2 is structured from a main unit portion 3, made out of metal, and a sensor portion 4, where a pair of barrier diaphragms (pressure bearing diaphragms) 5a and 5b, which are pressure bearing portions, is provided on a side face of the main unit portion 3, and the pressure sensor chip 1 is incorporated in the sensor portion 4.
In the meter body 2, the pressure sensor chip 1 that is incorporated in the sensor portion 4 is connected to the barrier diaphragms 5a and 5b that are provided in the main unit portion 3 through respective pressure buffering chambers 7a and 7b, which are separated by a large-diameter center diaphragm 6, and pressure transmitting media 9a and 9b, such as silicone oil, or the like, are filled into connecting ducts 8a and 8b, which connect the pressure sensor chip 1 to the barrier diaphragms 5a and 5b. 
Note that the pressure transmitting medium, such as the silicone oil, is required because it is necessary to separate the strain (pressure)-sensitive sensor diaphragm from the corrosion-resistant pressure bearing diaphragms, in order to prevent foreign materials within the measurement medium from becoming adhered to the sensor diaphragm, and to prevent corrosion of the sensor diaphragm.
In this differential pressure sensor 100, a first fluid pressure (first measurement pressure) Pa from a process is applied to the barrier diaphragm 5a, and a second fluid pressure (second measurement pressure) Pb, from the process, is applied to the barrier diaphragm 5b, as in the operating state during proper operation that is illustrated schematically in FIG. 11(a). As a result, the barrier diaphragms 5a and 5b undergo dislocation, and the pressures Pa and Pb that are applied thereto are directed to the first face and the other face of the sensor diaphragm of the pressure sensor chip 1, by the pressure transmitting media 9a and 9b, through pressure buffering chambers 7a and 7b that are divided by the center diaphragm 6. The result is that the sensor diaphragm of the pressure sensor chip 1 undergoes dislocation in accordance with the pressure differential ΔP between the pressures Pa and Pb that are directed thereto.
In contrast, if, for example, an excessively large pressure Pover is applied to the barrier diaphragm 5a, then, as illustrated in FIG. 11(b), the barrier diaphragm 5a undergoes a large dislocation, and the center diaphragm 6 undergoes dislocation in accordance therewith so as to absorb the excessively large pressure Pover. Given this, the barrier diaphragm 5a bottoms out on the bottom face (an excessive pressure guard face) of a recessed portion 10a of the meter body 2, controlling the dislocation thereof, and preventing the propagation of a greater differential pressure ΔP than that to the sensor diaphragm through the barrier diaphragm 5a. When an excessively large pressure Pover is applied to the barrier diaphragm 5b as well, as with the case wherein an excessively large pressure Pover is applied to the barrier diaphragm 5a, the barrier diaphragm 5b bottoms out on the bottom face (an excessive pressure guard face) of a recessed portion 10b of the meter body 2, controlling the dislocation thereof, and preventing the propagation of a greater differential pressure ΔP than that to the sensor diaphragm through the barrier diaphragm 5b. The result is that breakage of the pressure sensor chip 1, that is, breakage of the sensor diaphragm in the pressure sensor chip 1, due to the application of an excessively large pressure Pover is prevented in advance.
In this differential pressure sensor 100, the pressure sensor chip 1 is enclosed within the meter body 2, thus making it possible to protect the pressure sensor chip 1 from the outside corrosive environment, such as the process fluid. However, because the structure is one wherein the center diaphragm 6 and the recessed portions 10a and 10b are provided for controlling the dislocation of the barrier diaphragms 5a and 5b to protect the pressure sensor chip 1 from excessively large pressures Pover thereby, the dimensions thereof unavoidably must be increased.
Given this, there has been a proposal for a structure for preventing breakage/rupture of the sensor diaphragm through preventing excessive dislocation of the sensor diaphragm, when an excessively large pressure is applied, through the provision of a first stopper member and a second stopper member, and having recessed portions of the first stopper member and the second stopper member face the one face side and the other face side of the sensor diaphragm. See, for example, Japanese Unexamined Patent Application Publication No. 2005-69736 (“the JP '736”).
FIG. 12 illustrates schematically a pressure sensor chip that uses the structure illustrated in the JP '736. In this figure, 11-1 is a sensor diaphragm, 11-2 and 11-3 are first and second stopper members that are bonded with the sensor diaphragm 11-1 interposed therebetween, and 11-4 and 11-5 are first and second pedestals to which the stopper members 11-2 and 11-3 are bonded. The stopper members 11-2 and 11-3 and the pedestals 11-4 and 11-5 are structured from silicon, glass, or the like.
In this pressure sensor chip 11, recessed portions 11-2a and 11-3a are formed in the stopper members 11-2 and 11-3, where the recessed portion 11-2a of the stopper member 11-2 faces the one face of the sensor diaphragm 11-1, and the recessed portion 11-3a of the stopper member 11-3 faces the other face of the sensor diaphragm 11-1. The recessed portions 11-2a and 11-3a have surfaces that are curved along the dislocation of the sensor diaphragm 11-1, where pressure guiding holes 11-2b and 11-3b are formed at the apex portions thereof. Pressure introducing holes 11-4a and 11-5a are formed in the pedestals 11-4 and 11-5 as well, at positions corresponding to those of the pressure guiding holes 11-2b and 11-3b of the stopper members 11-2 and 11-3.
When such a pressure sensor chip 11 is used, then when there is a displacement of the sensor diaphragm 11-1 when an excessively large pressure is applied to the one face of the sensor diaphragm 11-1, the entirety of the dislocated face is supported and stopped by the curved surface of the recessed portion 11-3a of the stopper member 11-3. Moreover, then when there is a displacement of the sensor diaphragm 11-1 when an excessively large pressure is applied to the other face of the sensor diaphragm 11-1, the entirety of the dislocated face is supported and stopped by the curved surface of the recessed portion 11-2a of the stopper member 11-2.
This effectively prevents accidental rupturing of the sensor diaphragm 11-1 due to the application of an excessively large pressure, through preventing excessive dislocation when an excessively large pressure is applied to the sensor diaphragm 11-1, by preventing a concentration of stresses on the peripheral edge portion of the sensor diaphragm 11-1, thus enabling an increase in the excessively large pressure guard operating pressure (withstand pressure). Moreover, in the structure illustrated in FIG. 10, the center diaphragm 6 and the pressure buffering chambers 7a and 7b are eliminated, and the measurement pressures Pa and Pb are guided directly from the barrier diaphragms 5a and 5b the sensor diaphragm 11-1, thus making it possible to achieve a reduction in the size of the meter body 2.
However, in the structure of the pressure sensor chip 11 illustrated in FIG. 12, the entirety of the faces of the peripheral edge portions 11-2c and 11-3c of the stopper members 11-2 and 11-3 are bonded to the one face and the other face of the sensor diaphragm 11-1. That is, the peripheral edge portion 11-2c that surrounds the recessed portion 11-2a of the stopper member 11-2 faces one face of the sensor diaphragm 11-1, and the entire region of this oppositely-facing peripheral edge portion 11-2c is bonded directly to the one face of the sensor diaphragm 11-1. Moreover, the peripheral edge portion 11-3c that surrounds the recessed portion 11-3a of the stopper member 11-3 faces the other face of the sensor diaphragm 11-1, and the entire region of this oppositely-facing peripheral edge portion 11-3c is bonded directly to the other face of the sensor diaphragm 11-1.
With this structure, when an excessively large pressure that exceeds the excessively large pressure guarding operation pressure (the withstand pressure) by the stopper member 11-2 is applied, then after the sensor diaphragm 11-1 flexes to arrive at the bottom of the recessed portion 11-2a of the stopper member 11-2, in this state the sensor diaphragm 11-1 further flexes along with the stopper member 11-2. Given this, there is a problem that the vicinity of the edge (the position surrounded by the dotted line in FIG. 12) of the sensor diaphragm 11-1 on the side to which the pressure is applied, where the greatest tensile stress is produced, will be in a constrained state on both sides, thus causing a concentration of stress at that location, making it impossible to secure the expected withstand pressure.
Furthermore, when there is a mismatch, in manufacturing, in the opening sizes of the recessed portions 11-2a and 11-3a of the stopper members 11-2 and 11-3, there will be misalignment of the locations of constraints on the sensor diaphragm 11-1, with the effect thereof sometimes causing more pronounced concentration of stresses. In this case, the concentration of stresses is much more severe following the sensor diaphragm 11-1 arriving at the bottom, presenting the risk of a further reduction in withstand pressure.
The present invention was created in order to solve such a problem, and an aspect thereof is to provide a pressure sensor chip able to secure the expected withstand pressure by reducing the stresses due to constraints on the diaphragm, to prevent the concentration of stresses at the diaphragm edges.